Low-temperature transport properties of Ta_xN thin films (0.72 ≤ x ≤ 0.83)

We report on low-temperature (4-320 K) transport properties of Ta _xN thin films deposited on an amorphous SiO₂ substrate. In this work, Ta_xN thin films were restricted to a narrow range of x: 0.72 ≤ x ≤ 0.83 yet show considerable and nonmonotonic variation of their transport properties with Ta concentration. This behaviour is consistent with a local minimum in the density of electronic states at the Fermi level, as calculated for the rock salt intermetallic Ta4N5, and a rigid band model for describing the transport. The temperature dependence of the resistivity is best fit to the unusual form exp(-T/T₀). Interestingly enough, the fit parameter T₀ correlates well with the temperature of the maximum of the corresponding thermopower. Both of these characteristics, the fit and the correlation with the thermopower, are consistent with the Jonson-Mahan many-body formalism for charge and thermal transport when one has a nontrivial temperature dependence of the chemical potential. At the lowest temperatures measured, we have also found that the resistivity and thermopower show signatures of electron-electron interactions. We discuss also our results in the light of some theories usually used for describing transport of thin films and to other experimental investigations that have been performed on Ta _xN.